NAT

NAT is Network Address Translation. This is router or firewall translate one IP to another. Usually, is translate private address to public address.

Dynamic NAT

Dynamic NAT use a group of public IP address, doesn't make the mapping to the public IP.

Static NAT

Static NAT provides a one-to-one internal to public static IP address mapping.

PAT

PAT is Port Address Translation. This is all devices from inside network is presented as the same IP address in the outside, but use a different source port.

ALG

ALG is an Application Layer Gateway. The ALG modifies the SIP headers and SDP before the packet is forward to the upstream device.